Congestion Control
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Congestion Control



Unit 4 Of ACN

Unit 4 Of ACN



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    Congestion Control Congestion Control Presentation Transcript

    • Congestion Control
      • When one part of the subnet (e.g. one or more routers in an area) becomes overloaded, congestion results.
      • Because routers are receiving packets faster than they can forward them, one of two things must happen:
        • The subnet must prevent additional packets from entering the congested region until those already present can be processed.
        • The congested routers can discard queued packets to make room for those that are arriving.
    • Factors that Cause Congestion
      • Packet arrival rate exceeds the outgoing link capacity.
      • Insufficient memory to store arriving packets
      • Bursty traffic
      • Slow processor
    • Congestion Control vs Flow Control
      • Congestion control is a global issue – involves every router and host within the subnet
      • Flow control – scope is point-to-point; involves just sender and receiver.
    • Congestion Control, cont.
      • Congestion Control is concerned with efficiently using a network at high load.
      • Techniques for detection and recovery are:
        • Warning bit
        • Choke packets
        • Load shedding
    • Warning Bit
      • A special bit in the packet header is set by the router to warn the source when congestion is detected.
      • The bit is copied and piggy-backed on the ACK and sent to the sender.
      • The sender monitors the number of ACK packets it receives with the warning bit set and adjusts its transmission rate accordingly.
    • Choke Packets
      • A more direct way of telling the source to slow down.
      • A choke packet is a control packet generated at a congested node and transmitted to restrict traffic flow.
      • The source, on receiving the choke packet must reduce its transmission rate by a certain percentage.
      • An example of a choke packet is the ICMP Source Quench Packet.
    • Hop-by-Hop Choke Packets
      • Over long distances or at high speeds choke packets are not very effective.
      • A more efficient method is to send to choke packets hop-by-hop.
      • This requires each hop to reduce its transmission even before the choke packet arrive at the source.
    • Load Shedding
      • When buffers become full, routers simply discard packets.
      • Which packet is chosen to be the victim depends on the application and on the error strategy used in the data link layer.
      • For a file transfer, for, e.g. cannot discard older packets since this will cause a gap in the received data.
      • For real-time voice or video it is probably better to
      • throw away old data and keep new packets.
      • Get the application to mark packets with discard priority.